Significant portions of known oil reserves are bound in formations requiring enhanced oil recovery (EOR) techniques for effective and efficient extraction. Such reserves exist in previously untapped fields as well as in fields where traditional oil recovery has reached a practical limit. Among EOR techniques are steam-driven removal, typical of which is the so-called Steam-Assisted Gravity Discharge (SAGD) technique, and non-steam driven approaches. Non-steam driven approaches include water flooding to wash oil from oil-bearing formations by injecting large volumes of water into the formations and extracting the resulting oil-water mixture topside for processing. Generally, even after such EOR techniques as water flooding have been used on a field at least 40% of the Original Oil in Place (OOIP) remains in the formation.
Chemical flooding, another non-steam driven technique, has been found useful in extracting additional oil after other techniques have reached their practical limits as well as in some virgin fields. Practical limits often are based on limited water supply. While chemical flooding utilizes water, the chemical treatments reduce water requirements while increasing recovery of oil. Chemicals used for such treatments include polymers, surfactants, and alkali. While these chemicals may be used separately in aqueous solutions, considerable experience has developed in using them in combination in aqueous solutions. Such combination treatments are sometimes referred to as Alkali-Surfactant-Polymer (ASP) treatments or sometimes Alkali-Polymer (AP) treatments. For some fields, such treatments have been observed to result in an additional 15% to 30% extraction of the OOIP in the formation.
The oil-water mixture extracted from the chemically flooded formation is processed to separate the oil the oil-water mixture to produce an oil product stream and a produced water stream. Just as in other water-based EOR processes, reuse of the produced water is desirable. However, the total suspended solids (TSS), hardness and other scale formers, and residual oil and grease in the produced water present challenges in recycling the water. Scaling of infrastructure and well pore plugging concerns make it advantageous to effectively remove TSS, often desirable to sizes below 1 micron, hardness and other scale formers, and residual oil and grease from the produced water before reusing the water. A particular challenge in treating EOR produced waters is the fact that much of the oil contaminants are in emulsified form. Emulsified oil droplets typically range from 0.5 and 20 micron in size and differ from less challenging free floating oil with droplets size typically>100 micron and dispersed phase oil with droplet size typically 20 to 100 micron. Free floating and dispersed phase oil droplets can largely be removed by gravity separation such as skimming and gas floatation techniques, but emulsified oils often require more complex conventional treatment that often still results in poor effluent quality. There exists a need for improved approaches to cleaning produced water in chemical flooding EOR operations.